More precisely the present invention relates to closure devices having complementary profiles 10′, 20′ borne by respective opposite support webs or walls 12′, 22′ such as illustrated by way of non-limiting example in the appended FIG. 1. Many variant embodiments of the device illustrated in FIG. 1 have already been proposed and fall within the scope of the present invention.
Generally, the abovementioned complementary profiles are formed from complementary male/female elements, the male element comprising a rod 14′ secured to the web or to the wall 12′ and provided with an widened protrusion 15′ at its top, while the female element 20′ can be formed by a groove 24′ with convergent edges having as a section a complementary cavity of the protrusion 15′.
Closure devices comprising several parallel complementary profiles also have been proposed to improve the closing and the sealing of the bags to which these devices are fixed.
Yet the applicant has noticed that when the closure devices comprised multiple profiles, and in particular complementary profiles of male/male or female/female type, especially in the form of an arrow, coextrusion of the complementary profiles with their support web and the film intended to form the walls of the bag tends to deform the closure devices, in particular when the device is formed by means of blown film extrusion.
The blown film extrusion method consists of extruding material, generally thermoplastic, through an annular die then blowing it to form a jacket which is then cooled and rolled up.
An embodiment of a manufacturing machine by means of blown film extrusion is illustrated in FIG. 8. During a first step, plastic material, generally based on polyethylene, is introduced to an extruder.
During a second step, the plastic material is heated and extruded so as to plasticize it and form via an annular die a film of annular form.
During a third step, air is blown inside the film to inflate it and form a jacket (or bubble). The inflation rate (ratio between the diameter of the jacket after inflation and the diameter of the film leaving the die) is generally comprised between 1.5 and 3.5 to obtain good extrusion qualities. Air is also blown onto the jacket from the outside of the latter by a ring system to cool the extruded and blown film.
During a fourth step, the film is flattened using nip rollers.
This method produces very fine films. But when the film, which is intended to form the walls of a bag, is coextruded with a closure device comprising several complementary profiles, at the exit of the die, necking of the closure device differs from necking of the film. This different necking creates stresses at the interface, especially between the closure device and the film, the effect of which is retracting or dilating the support webs and therefore moving apart or bringing together the complementary profiles to or from each other respectively.
This deformation effect therefore creates spaces between the complementary profiles during their engagement, likely to let through fluids and/or fine particles contained in the bag, or to the contrary bring them together to such a point where the profiles stick together. Also, the resulting closure devices are more difficult to close by mutual engagement of the associated complementary profiles, and risk opening more easily.
For some bags it is necessary to make closure devices with multiple complementary profiles, integrally formed with the film and preferably via blown film extrusion. This is the case especially of closure devices configured to generate a tactile sensation and a sound effect during closing and opening, that is, respectively during engagement and separation of the profiles. An example of such devices is described in document WO 2013/076120. In particular this document describes a closure device for bags or the like comprising two support webs or walls 12′ provided on their internal face with a complementary closing assembly comprising a series of complementary profiles 10′ which extend in parallel in a main direction extending according to a longitudinal axis (FIG. 2). At least one of the complementary profiles 10′ has sequential alterations 11′ according to a direction perpendicular to the support webs. These sequential alterations can comprise sequential crushing of the engaged profiles, such that the sequential crushing effect flares both complementary profiles in the direction of the width transversal to the elongation direction of the closure device, incisions without removal of material, optionally stretched to form cuts, and/or cuts with removal of material.
In practice it proves necessary to make these closure devices by means of blown film extrusion, this method easily producing, and at reasonable cost, bags comprising a closure device integrally formed and in a single piece with its walls. This manufacturing method in fact prevents report of the closing assemblies after the walls of the bag have been made.
It is therefore possible to correct the form of the die of the extruder to anticipate the deformations of the profiles due to inflation of the jacket. But the applicant has noticed that such correction was not enough to ensure engagement of the complementary profiles. In fact, it turns out that the complementary profiles obtained using a corrected die tend to stick to each other when correction is done so as to bring them together by anticipation of inflation of the film, or to the contrary stick to the film if the correction consists of moving them apart.